This research aims at the development of a combination lutetium oxyorthosilicate and yttrium oxyorthosilicate (LSO/YSO) detector for use in a novel high-performance combination positron emission tomography and single-photon emission computed tomography (PET/SPECT) system. Due to the basic characteristics of the detector materials, simulations indicate that such a combination detector will yield tomograph performance equal to the present state-of-the-art SPECT tomographs and will provide an improvement in PET countrate greater than a factor of 20 over existing SPECT tomographs with coincidence. YSO has single-photon detection properties (e.g., density, mean free path, light output, and decay time) that compare favorably with those of thallium-doped sodium iodide (NaI(Tl)), the scintillator in most SPECT systems. Thus, the use of YSO for single photon detection appears feasible. LSO has detection properties that are generally better than those of bismuth germanate (BGO), the scintillator in most PET systems. In particular, the light output of LSO is approximately 5 times greater than that of BGO and the decay time one-seventh as long as that of BGO. Thus, the use of LSO in PET systems is likely. LSO and YSO also have properties that support their use in combination. Investigations of the characteristics of phoswiches of LSO and YSO and of a detector comprising such a phoswich and four photomultiplier tubes (PMTs) are proposed. The specific aims of the proposed research are the design, fabrication, characterization, and evaluation of a prototype LSO/YSO phoswich detector for use in novel PET/SPECT systems. PROPOSED COMMERCIAL APPLICATIONS: It is believed that a cost-effective (approximately the cost of a SPECT tomograph with coincidence) commercial PET/SPECT tomograph will result if the proposed detector is proven feasible. The performance of the tomograph is expected to be equivalent to existing SPECT and existing Research PET tomographs. There are approximately 800 dual headed SPECT tomographs acquired by the industry each year.